Spray Pyrolytic Deposition of Zirconium Oxide Thin Films: Influence of Concentration on Structural and Optical Properties

¹Nanostructured Thin Film Materials Laboratory, Department of Physics,Govt. Vidarbha Institute of Science and Humanities, Amravati - 444604, India

²Advanced Physics Laboratory, Department of Physics, Savitribai Phule Pune University, Pune - 411007, India

³Sinhgad Institute of Technology, Lonavala - 410401, India

*E-mail: ashokuu@yahoo.com

1. Introduction

Zirconium oxide thin films have acquired considerable attention in the variety of applications such as protective coatings, insulating dielectric layers, catalysts, sensors and fuel cells as the properties of the films are found to be dependent on the growth conditions and methods of synthesis. Many efforts have been put in the development of low cost techniques which can offer the desired optical and structural characteristics of the material. ZrO has been rigorously studied from 2 the past few decades because of its distinct properties like high chemical inertness, low thermal conductivity, high dielectric constant, high transparency in the visible and near infrared region and high refractive index.1, 2 Several methods have been employed for the deposition of ZrO thin films which include sputtering,3 spin coating,4 2 sol–gel,5 radio-frequency sputtering,6 pulsed laser deposition,7 metal–organic chemical vapour deposition8 and spray pyrolysis.9 However, spray pyrolysis has become one of the preferred methods for the deposition of thin transparent oxide layers due to some of its distinct advantages such as lower crystallization temperature, homogeneity,simplicity, excellent compositional control and cost effectiveness.

Ruiz et al.10 have obtained zirconia films by spray pyrolysis technique and investigated the effects of composition of substrate (carbon steel, stainless steel, alumina and borosilicate glass), time (1 and 2 h), temperature (450 oC and 650 oC), and carrier gas (Ar, N, He and air) on characteristics of the films. Stelzer and Schoonman11 have deposited zirconium oxide films by spray pyrolysis using Zr- acetylacetonate [(Zr(C H O ) ] as precursor. Peshev et al.12 have used 5 5 2 4 oxalic acid and zirconyl chloride octahydrate to prepare a precursor solution. The study showed that the spray pyrolytic deposited films on silica substrate were amorphous in nature. On heat treatment, cubic zirconia phase was detected in the films at temperatures T = 500-700 oC. Both cubic and monoclinic phases were detected as temeperature rises from 700 - 1000 oC. The c-ZrO content decreases with increase in 2 temperature. Ortiz et al.13 have deposited zirconium oxide films onto fused quartz and silicon in the temperature range of 300 C to 575 oC by the pyrosol process using precursor solution of zirconium acetylacetonate. The films were found to be amorphous in nature below 425 oC. The cubic crystalline phase was observed for short deposition times and higher temperatures. The films showed monoclinic crystalline phase for long deposition times. Ramos-Guerra et al.14 have studied structural and photoluminescence properties of un-doped and ZrO films 2 doped with Tb3+ and Eu3+ ions by ultrasonic spray pyrolysis technique. The XRD study demonstrated the polycrystalline nature and the existence of tetragonal metastable phase of the films. They revealed that the deposition temperature decides the surface morphological characteristics of the films. The photoluminescence analyses of the films presented a broad emission peak associated with radiative transition within ZrO matrix at 440 nm as well as characteristic emission peaks 2 associated with Eu3+ and Tb3+ ions. Also, it was noticed that the intensity of the photoluminescence emission increases with increase in deposition temperature. Jothibas et al.15 have deposited ZrO thin films at various 2 substrate temperatures in the range 275-475 oC. The XRD study demonstrated that the crystallite size of the films increases with increase in substrate temperature. Further, the study confirmed the tetragonal phase of ZrO . The UV-vis spectroscopic analyses revealed that with 2 increase in the substrate temperature, the band gap value of the film increases.

Depending on temperature and atmospheric pressure conditions, ZrO 2 exists in three polymorphs: monoclinic (m-ZrO ) phase which the stable 2 phase from room temperature up to 1100 oC, tetragonal (t-ZrO ) phase 2 in the range 1100 oC to 2300 oC and cubic (c-ZrO ) phase above 2300 2 oC.12 However, at low temperatures, the existence of both tetragonal and cubic metastable phases has been reported by number of research groups. These polymorphic forms depend upon dopants, the presence of impurities, the particle size, the preparatory conditions and precursors.

The present study reports the deposition of ZrO thin films on glass 2 substrates by spray pyrolysis technique. The effects of concentration of precursor solution on the structural and optical properties of ZrO films 2 were investigated.

2. Experimental section 

The materials for thin film deposition were zirconyl chloride octahydrate (ZrOCl .8H O) and distilled water by SRL, Mumbai. The 2 2 ZrO films were deposited using ZrOCl .8H O dissolved in distilled 2 2 2 water. The precursors were kept ready by varying molarity of solution, thus, four solutions, labeled as C1, C2, C3 and C4 (see Table 1). The flow rate of the carrier gas was 35 L/min. The distance between the substrate and nozzle was 25 cm. The compressed air was used as a carrier gas to spray the precursors at the rate of 7.5 ml/min. The tin bath temperature was maintained as 450 oC and the glass substrates were kept on it. Fig. 1 shows the schematic of spray pyrolysis technique.

The crystal structure and crystallinity of the deposited films were investigated by X-ray diffraction (XRD). The patterns were obtained on Rigaku ''D/B max-2400'' ( λ = 1.54 Å) X-ray diffractometer. The surface morphology of the deposited films was observed by scanning electron microscopy (SEM, JEOL-JSM 6360-A). The compositions of the deposited metal oxide films were analyzed by scanning electron microscope (SEM, JEOL-JSM 6360-A) equipped with an energy dispersive X-ray (EDX) detector. Transmission electron microscopic (TEM) investigations were conducted on PHILIPS CM 200 (Operating voltage: 200 kv, Resolution: 2.4 Å) microscope. The optical properties of the ZrO thin films were studied in the wavelength region of 2 200–800 nm using UV–vis spectrometer (JascoV-670). The FT-IR spectra of the ZrO thin films were recorded in the range from 400 to 2 4000 cm-1 in transmittance mode using FT-IR-6100 type A spectrometer.

3. Results and Discussion

3.1 Structural analysis

The XRD patterns of ZrO thin films deposited onto glass substrate at 2 450 oC at various concentrations of precursor solution are shown in Fig. 2. The XRD pattern of the as-deposited ZrO film (Fig. 2a) shows that it 2 is amorphous in nature which indicates that the imposing material does not have adequate concentration to produce a crystalline phase. Xiaming et al.16 have reported that ZrO molecules are formed during 2 the pyrolytic reaction without intrinsic close links at the same substrate temperature. This is an amorphous phase which is very much close to cubic phase of ZrO . During crystallization, firstly, metastable c-ZrO is 2 2 obtained and then depending upon the preparatory conditions it gets transformed to m-ZrO or t-ZrO . In Fig. 2b-d, it is seen that in case of 2 2 C2 sample crystallization has begun which increases with increase in concentration. The four major peaks corresponding to (111), (200), (220) and (311) planes confirm the cubic crystal structure in case of all the ZrO thin film samples.10, 12, 14, 21 All these polycrystalline ZrO films 2 2 are free from impurities as no impurity peaks are observed. The positions of all the peaks are in accordance with the standard ZrO data 2 (JCPDS: 27-0997) (see Table 2). The intensity of above peaks increases with increasing concentration of solution up to 0.1 M due to the improvement in crystallinity of the films. The average crystallite size (D) was calculated by the Scherrer formula17 using the full width at half maximum (FWHM) of the most intense peak for different samples,

Where, β is the FWHM in radians, λ is the wavelength used and θ is the Bragg's angle. The crystallite size of C2, C3 and C4 is 12 nm, 9 nm and 11 nm, respectively. To find the defects in the nanocrystalline ZrO 2 films, the dislocation density was calculated using the relation,18

The dislocation density was calculated for the given crystallite size. The dislocation density is high for small crystallite due to more number of interfaces in the given volume. The micro strain of the sample was also estimated using the equation,18

The micro strain, dislocation density and crystallite size for nanocrystalline ZrO films are listed in Table 3.The existence of 2 defects around the lattice could be known from the micro strain. From the observations, it can be concluded that the re-crystallization process in case of polycrystalline films leads to changes in δ and ε.15, 19

3.2 Surface morphological and elemental analysis 

Fig. 3 shows SEM images of ZrO films deposited at different molar 2 concentrations. D. Perednis et al.20 have reported that chlorine impurities hinder the crystallization and hence, initially, the films are amorphous in case of chlorides. For C1, a thin and transparent layer was deposited on the glass substrate. The layer was free from cracks. On the other hand, slightly larger cracks are seen in the films for C4 (0.1 M) solution (Fig. 3d) compared to the C1 (0.025 M), C2 (0.05 M) and C3 (0.075 M) salt solutions (Fig. 3a-c). With increase in the concentration of precursor solution, the particle size increases on drying. The spreading of droplets could be the deciding parameter to bring the changes in surface morphology of the films. The roughness of the films is strongly influenced by the rate at which the liquid droplets spread. As viscosity of solution increases that decreases the spreading rate of liquid droplets. In case of the droplets of C1, C2, C3 and C4 concentrated solutions, the spreading of droplets decreases due to increase in viscosity. If rapid drying process is implemented that increases the surface roughness of the films due to incomplete spreading of liquid droplets. It can be concluded that for the deposition of smooth and dense films the concentration of spraying solution should not be too high. However, in order to achieve high deposition rates, a high salt concentration is preferred. It was observed that the increase in salt concentration in the precursor solution increases the surface roughness of the deposited film. The EDX spectra show the presence of Zr and O (Fig. 4a-c).

The size and morphology of zirconia samples deposited at 450 oC were further explored with TEM analyses. Fig. 5a-c show influence of concentration of solution on TEM images of the ZrO coatings. The 2 TEM images confirm that the flakes consist of agglomerates of ZrO 2 nanoparticles with the average particle size < 20 nm. The selected area electron diffraction (SAED) pattern shows that the formed layer was composed of crystalline ZrO nanoparticles (Fig. 5d-f ). Further, the

 

SAED patterns give spot pattern. However, it seems it is not a single pattern, but a more complex pattern which could be caused by aggregates of nanoparticles. The d-spacing corresponding to the diffraction rings of the SAED patterns is in accordance with the cubic phase of ZrO nanocrystals. The SAED patterns of the aggregates of 2 ZrO nanoparticles confirm the nanocrystallinity of the films. The 2 concentric Debye–Scherrer rings can be indexed to the (111), (200), (220) and (311) planes and are assigned to the cubic phase. The results show that no phase transformation occurs with increase in concentration of precursor solution. The crystallite size obtained from the X-ray diffraction and TEM results are in good agreement.

3.3 UV-vis analysis

The optical absorption measurements were carried out in the wavelength region of 200-800 nm to estimate the optical band gap energy. The optical band gap (E ) in ZrO thin films was estimated using g 2 the relationship between the absorption coefficient (α) and the photon energy (hυ) given by the equation

where A is a constant, is absorption coefficient, E is the band gap g α energy, and n is equal to 2 for indirect and ½ for direct transition. Fig. 6 shows the plot of ( h ) 2 versus h of ZrO thin films deposited on glass 2 α υ υ substrate for different concentration of precursor solution. The optical band gap was determined by extrapolating the straight line portion of the plot to the energy axis for α = 0.21 The optical band gap values were found to be 4.18 eV (for 0.1 M concentration), 4.03 eV (for 0.075 M and 0.05 M) and 4.05 eV (for 0.025 M). It has been observed that the optical band gap deceases with decrease in concentration of precursor solution. The decrease in optical band gap of the films could be attributed to changes in structural defects, grain size and atomic distances. The improvement in crystallinity and morphology of the film decreases the optical band gap of the material with increase in concentration of precursor solution.22, 23 The inset of Fig. 6 shows the optical absorption spectra of ZrO thin films deposited on glass substrate

at different concentration of precursor solution at 450 oC. In visible region, the optical absorption spectra revealed the low absorbance characteristic feature of ZrO . 

3.4 FTIR analysis 

Fig. 7 shows influence of concentration of precursor solution on the FTIR spectra of zirconia films. The C1 film sample exhibited O-H stretching vibration in the frequency range 3406-3908 cm-1 (see Fig. 7a). This could be due to the interaction between Zr4+ and inner hydroxyl groups. This interaction builds superior force of attraction on the stretching vibration due to reduction in the vibrational dipole moment. Hence, in case of C2, C3 and C4 zirconia film samples the structural vibration for O-H were appeared at wave number greater than 3000 cm-1 (see Fig. 7b-d and refer Table 4). The O-H stretching vibrations were observed at 3980, 3783, 3595 and 3461 cm-1 in case of C2. In C3 sample, the O-H stretching vibrations were appeared at 3962, 3792, 3658 and 3587 cm-1 whereas at 3968, 3778, 3652 and 3601 cm-1 for C4.24, 25 The vibrations appeared at 2941, 2932, 2923 and 2937 cm-1 for C1, C2, C3 and C4 respectively which could be ascribed to the impurities in the form of organic residues.26 The bands appeared at 2296 cm-1 for C1, 2296 cm-1 for C2, 2306 cm-1 for C3 and 2358 cm-1 for C4 can be attributed to adsorbed CO vibrations.27 The bands observed at 2 1589 cm-1 for C1, 1598 cm-1 for C2, 1582 cm-1 for C3 and 1549 cm-1 for C4 could be due to the N-O asymmetric stretching vibration. For all samples, the vibration appeared at 1428 cm-1 could be ascribed to C-C stretching vibration.28, 29 The C-N stretching vibrations were observed at 1024 cm-1 for C2, 1030 and 1091 cm-1 for C3 and 1021 and 1092 cm-1 for C4. For C1 and C2, O-H bending vibration was appeared at 899 and 944 cm-1 respectively.26- 29

4. Conclusions 

A precursor solution of zirconyl chloride octahydrate (ZrOCl .8H O) 2 2 can be used to deposit zirconium oxide films by spray pyrolysis technique. The influence of concentration of precursor solution on the structural and optical properties of ZrO films were investigated by 2 XRD, SEM, EDX, TEM, UV- vis and FT-IR techniques. XRD analyses demonstrated that the films prepared at lower concentration (0.025 M) were found to be in amorphous phase. The increase in concentration from 0.05 M to 0.1 M of precursor solution increases the crystallinity of the films. Only cubic zirconia (c-ZrO ) phase was indentified in all the 2 crystalline film samples at substrate temperature of 450 oC. TEM studies showed that the average particle size in all film samples was < 20 nm. The SAED patterns showed that films composed of crystalline cubic ZrO nanocrystals. The optical band gap values of the deposited films 2 were increased with increase in concentration of precursor solution. The formation of ZrO was further confirmed by FT-IR studies.

Conflicts of interest

There are no conflicts to declare

Acknowledgements

Authors are thankful to Board of College and University Development (BCUD), Savitribai Phule Pune University, Pune for financial support under the project.